The problem of counting people traffic with beams of light is known. Typically, people counters are used at doorways in places of public accommodation such as stores and other buildings to roughly count occupancy and correspondingly control ventilation, heating and air conditioning systems. People counts have other purposes as well; in retail establishments, people counters may be used in store aisles or other locations to determine interest in those particular areas, and may be used to generate statistics such as total traffic through a store or particular aisle, and to perform data mining when combined with other data, e.g., by using register transaction counts to test the efficiency with which sales are being consummated from visiting potential customers in the store or particular aisles.
The most common approach to people counting has been to produce a light beam across a passageway, to count the number of persons passing through the passageway, represented by the number of times the light beam is broken.
A battery powered people counting system that uses this broken-beam approach is described in U.S. patent application Ser. No. 10/635,403, filed by the applicant hereof. This patent application describes an object counter that uses an infrared (IR) light source that generates and detects brief pulses, using very fast emitter/sensor devices and reducing the data cycle to approximately 20 microsecond of IR emission for every 1/16-second operation. This is a power on-to-off ratio of approximately 1 to 300, permitting low power consumption and long-term battery-powered operation. The applicant's U.S. Pat. No. 6,721,546, which is hereby incorporated herein by reference, describes additional low-power techniques that use a processor for a brief period of time.
Single-beam people counters such as disclosed in the above patent, can readily track a beam break, but cannot readily determine the direction of movement of an object or a person that caused the beam break. When counting movements through separate entrance and exit doors in a building, the location of the beam indicates whether the person is entering or exiting. However, when monitoring a passageway that is bi-directional, or where a common door is used for entry and exit, a single beam is not typically able to discriminate between the entry of a person and the exit of a person. For such applications, therefore, it has been known to use a directional people counter.
Directional people counters a retro-reflective target and two narrow beam emitter/sensor assemblies to produce two physically separated beams. The beams must be narrow enough such that the two sensors do not see each other's beams as they are reflected back from the retro-reflective target. Referring to FIG. 1A, the physical arrangement of the beams in a typical prior art two-beam counting system 10 can be explained. The beams A and B from emitters EA and EB are launched across the entranceway 12 toward a retro-reflective target 14. The beams reflect from the target 14 and back toward the system 10 and sensors SA and SB positioned therein. (In FIGS. 1A and 1B, the scale of the distance between the emitters is exaggerated relative to the scale of the distance across the passageway being monitored.)
It is necessary in these typical directional object counting systems, that the emitted beam from the emitters EA and EB be sufficiently narrowly focused that, when mirror 14 is properly positioned, the respective beams A and B from EA and EB will illuminate only one of the corresponding sensors SA and SB. Thus, the shaded area in FIG. 1A, representing the region illuminated by beam B from EB, does not include sensor SA. Also, the unshaded area in FIG. 1A, representing the region illuminated by beam A from EA, does not include sensor SB. Similarly, the field of view of the sensors must be sufficiently narrow to exclude stray light emitted from the opposite emitter. Only when this condition is met will sensor SA and sensor SB working with emitters EA and EB create independent beams A and B across the passageway, which reflect the existence or absence of an object in two different regions of the passageway 12. When a person or object passes in direction 16, the object/person will break beam A first, which will cause a loss of signal at sensor SA, and then bream beam B, causing a loss of signal at sensor SB. Conversely, when a person or object passes in direction 18, beam B will break first, causing a loss of signal at sensor SB, and then beam A will break, causing a loss of signal at sensor SA.
Directional people counters thus detect direction of motion by the sequence in which beams are broken and signal lost at sensors. If direction 16 is the direction of entry and direction 18 is the direction of exit, then a break of beam 16 first means an entry, and a break of beam 18 first means an exit.
It will be noted that this method of dual-beam people counting requires optically precise emitters EA and EB, that emit a beam with a relatively narrow aperture angle α, and optically narrow field of view sensors, so that the field of view of sensor SA cannot see stray light from emitter EB emitter and the field of view of sensor SB cannot see stray light from emitter EA. If the field of view and aperture angle α of the sensor and emitter are excessively large for the application, then the beams A and B returning to sensors SA and SB will activate both sensors, as shown in FIG. 1B.
Typically the width of the passageway is several feet and the emitter-sensor center-to-center separation is only a few inches. As a result an emitter beam divergence of far less than 30 degrees would result in both sensors having a view of both emitters. In this circumstance, the signals received at the sensors SA and SB will be a function of the signals transmitted from both emitters EA and EB, and as a result, both beams EA and EB must be broken before either sensor will lose signal. Thus sensors SA and SB will lose signal simultaneously or nearly so, and only when both beams are broken, and it will be difficult to determine the direction of motion because the beam are not clearly and unambiguously broken at different times, as is the case when the beams have a sufficiently narrow aperture angle as shown in FIG. 1A.
The reason that the beam generating/sensing assemblies EA/SA and EB/SB are separated by only a few inches is that a smaller package is the better for object counting applications, as object counters are typically mounted on door frames and walls. Thus, the emitter-sensor separation, and the width of the passageway being monitored, are relatively fixed. As a result, beam emitter/sensor assemblies must have particularly narrow beams and fields of view for such applications. This means the beam generating and sensing assemblies are optically precise and complex in design, with a lens and collimator required to produce the small viewing angle necessary so that the sensors SA and SB do not see the beam from the opposite emitter EB and EA. These precision optics result in a high manufacturing cost.
The need for a narrow field of view sensor also increases power requirements on the IR emitter. The power emitted by the IR emitter, combined with the sensitivity of the IR sensor, determine the sensing range of the assembly. The more IR power emitted, the greater the range will be for a given IR emitter sensitivity. However, the requirements of a retro-reflective dual beam directional object counter, require sensors with a narrow field of view and a consequently lower sensor sensitivity. That lower sensitivity of the sensor, must be compensated by a higher power IR emitter to achieve a desired sensing range. The power required to operate such a system is typically too high for battery powered operation for reasonably long periods. As a result, wires must be used to supply electrical power to the beam counting system, and to communicate beam break sequence data to a location where it can be incorporated into a higher-level application such as retail traffic monitoring. Wiring costs are high in many installations and often contribute more to overall cost than the beam sensor.
It is an object of the present invention to provide an accurate directional people counting system that does not require precise optics and the attendant expense therefor, and which can operate on battery power for suitably long periods of time thus eliminating the need for wiring to a central location.